Sepsis patients often develop cytokine storms in the early phase, and immunoparalysis associated with lymphocyte apoptosis in the later phase of illness. The majority of sepsis-related deaths occur during this secondary hypo-immune state, often due to a failure to clear pathogens. Recent studies have implicated lymphopenia as an important component in the pathogenesis of sepsis-associated immunoparalysis. A prospective cohort study found that monocyte programmed death-ligand 1 (PD-L1) expression was associated with increased risk of mortality in septic patients. PD-L1 is an immune checkpoint protein. PD-L1 engagement with programmed cell death protein 1 (PD-1), also an immune checkpoint protein, on T lymphocytes can lead to lymphocyte apoptosis. Studies in animal models of sepsis have shown that blockade of PD-1 or PD-L1 attenuates lymphocyte apoptosis, alleviates organ damage, reduces bacterial burden, and decreases mortality. Mkp-1 is a negative regulator of both p38 and JNK, and serves to restrain the production of both pro- and anti- inflammatory cytokines (e.g., TNF-?, IL-6, and IL-10). In an E. coli-induced sepsis model, Mkp-1 knockout (KO) mice exhibit features of severe sepsis, including substantially enhanced cytokine storm, exacerbated organ damage, increased mortality, and elevated bacterial burden relative to wildtype (WT) mice. Septic Mkp-1 KO mice also exhibited increased mortality relative to septic WT mice in a cecal ligation puncture model. Notably, a clinic study has found that MKP-1 expression is suppressed in septic patients, highlighting the clinical relevance of the Mkp-1 KO mouse model of sepsis. While studies in our laboratory and by others suggest that excessive IL-6 and IL-10 in Mkp-1 KO mice are, at least partially, responsible for the compromised bacterial clearance, the mechanisms remain unclear. Recently, we have found that PD-L1 expression in E. coli-infected Mkp-1 KO mice is substantially higher than that in similarly infected WT mice. Moreover, PD-L1 expression is tightly correlated with IL-6, IL-10, and STAT3 expression. While PD-L1 can be induced by IFNs via the JAK/STAT pathway, type I & II IFN mRNA were barely detectable. Since IL-6 and IL-10 also regulate the JAK/STAT pathway, we postulate that in the later stage of sepsis IL-6, IL-10, and perhaps also circulating IFNs up-regulate PD-L1 expression via the JAK/STAT pathway. We further hypothesize that over-produced IL-6 and IL-10 or IFNs in septic Mkp-1 KO mice compromise immune defense by potentiating lymphocyte apoptosis via augmenting PD-L1 expression, ultimately increasing bacterial burden and mortality. In this proposal, we propose two Aims to test these hypotheses. Aim 1 will assess whether PD-L1 neutralizing antibody decreases bacterial burden and mortality in septic Mkp-1 KO mice. Aim 2 will assess whether IL-6 and IL-10 enhance PD- L1 expression and if JAK inhibitors attenuate PD-L1 expression, decrease bacterial burden, and increase animal survival in septic Mkp-1 KO mice. Successful completion of the Aims will reveal the mechanisms underlying PD-1 induction, and uncover a novel mechanism by which Mkp-1 protects immune competence.